cellsync: a displacement check waits for a branch already being kept

Displaced returned at once when the hold was already stale, but a background
heartbeat or follower sets stale before it finishes saving and branching the
kept work. CheckLease could then return before the branch map was written.
Displaced now joins lost under flushMu, whose own checks still prevent a
second resolution, so the caller only returns once the kept branch exists.
The window grew once displacement settles held calls before branching.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: AbirAbbas

internal/cellsync/batch.go

@@ -536,10 +536,10 @@ func (b *Batcher) beat(ctx context.Context) {
 // work is refused immediately and any pending turns are resolved under flushMu
 // so an in-flight publish and a watch wakeup never announce displacement twice.
 func (b *Batcher) Displaced(ctx context.Context) {
-	seen, _, stale := b.view()
-	if stale {
-		return
-	}
+	// A stale hold may still be saving its kept branch under flushMu.
+	// Joining lost waits for that resolution; the stale flag only retires
+	// ownership and cannot acknowledge that preservation has finished.
+	seen, _, _ := b.view()
 	b.lost(ctx, seen)
 }